This invention relates to a new process for bonding surfaces using novel hotmelt adhesive compositions.
Bonding processes using solventless adhesive systems containing 100% solids are currently acquiring increasing significance, in part because the use of solvent-based systems requires expensive solvent-recovery plants. Moreover, the use of water-based dispersion or solution systems requires the evaporation of the water, which can also be very expensive. An alternative is the use of hotmelt adhesives, which have been known for some time. An advantage of hotmelt adhesives is that, after application in the form of hot melts, they quickly solidify on cooling and hence strengthen. One disadvantage of hotmelts is related to their high melting temperatures. In particular, temperature-sensitive substrates are difficult to bond because the adhesive layer, unless it is further processed immediately, rapidly crystallizes into a state no longer having a wetting effect and which can only be thermally activated under extreme temperature conditions. In addition, the bond shows limited heat resistance because of the thermoplastic character of the hotmelt.
German Patentschrift 878,827 describes a bonding process in which an ester containing isocyanate groups dissolved in methylene chloride is applied to a surface and the coating is stored so that chain extension can take place. After the surface is then fitted to the other surface to be bonded, a highly elastic bond is established after application of light pressure and storage above 30.degree. C. Instead of using a dissolved hotmelt, it is also possible according to this patent to use solvent-free conditions.
A solvent-free hotmelt bonding process is specifically claimed in German Offenlegungsschrift 2,609,266, which describes a bonding process using reactive hotmelt systems based on isocyanate-containing prepolymers of diisocyanates and polyester diols having melting ranges above 40.degree. C. In view of their low molecular weights, the products are liquid, and thus can be processed, at temperatures just above the melting range of the polyesters. After a chain-extending reaction on the substrate, such liquids reach a sufficiently high molecular weight that, together with the recrystallization of the crystalline soft segments, provides increased initial strength. The final strength is achieved by complete reaction of the residual free isocyanate groups, for example, with atmospheric moisture, to form linear high molecular weight polyurethane polyureas. In view of the very fast cycle times typically encountered in industry, however, the necessary pre-extension of the isocyanate-containing prepolymers is difficult to reproduce and, if insufficiently complete, results in defective bonding caused by inadequate cohesive strength. If, on the other hand, pre-extension proceeds too far, the resultant adhesive layer can no longer be sufficiently activated because of an excessively high molecular weight.
Another method of producing reactive hotmelts is described in European Patent Application 340,906. However, the use of two polyesters having different glass transition temperatures makes the products expensive to produce and the viscosities of the products are relatively high because of the relatively high glass temperatures. Accordingly, the method requires a relatively high processing temperature for thorough wetting of the substrates. In addition, measured application of the hotmelts is problematical.
European Patent Application 354,527 describes hotmelts based on crystalline polyesters and polyisocyanates but does not mention suitable methods for their application.
Although the molten adhesive can be applied by rollers, the reactivity of the isocyanate groups with atmospheric moisture gradually leads to caking that is extremely difficult to remove, particularly where the machines have been in operation for prolonged periods.
Spraying processes are far more suitable, enabling the molten isocyanate-reactive hotmelts to be handled in the absence of moisture. Atmospheric moisture has access to the hotmelt only after it has been applied to the surface to be bonded. The adhesives, however, must satisfy certain rheological requirements to obtain a satisfactory spray pattern.
Accordingly, there exists a need for a bonding process that can be readily carried out on an industrial scale and allows a fast machine cycle time, even under adverse conditions, but nevertheless can be carried out with relatively low-melting adhesives capable of activation at moderate temperatures.